Get under your skin

In the case of sea cucumbers, the internal skeleton takes the form of tiny particles called ossicles, embedded in the outer skin. Taxonomists can identify sea cucumbers by examining the shape of their skin ossicles under a light microscope.

In addition to ossicles, M. intermedia has something else lurking inside its skin: tiny iron- and
phosphate-rich blobs called phosphatic bodies. You can see these microscopic orange blobs in the image to the right above. The animals get more of these as they get older. Scientists think these granules serve to strengthen the connective tissue in the skin.

No foot to stand on

A living Molpadia specimen with its feedingtentacles extended. This individual is waitingto be weighed and measured in the field.

Sea cucumbers typically have rows of tube feet running the length of their bodies; however, M. intermedia lacks tube feet, giving it a smooth and shiny appearance. Its soft, cylindrical body has muscles running its length. These muscles can expand to lengthen it (up to 43 cm) or contract to shorten it into a little ball when it is disturbed.

At one end of its body is a mouth surrounded by 15 feeding tentacles. These short finger-like (digitate) tentacles help push food into its mouth as it ingests sediment while burrowing. At the rear end of its body is a short, stubby tail.

Couch Potato

M. intermedia seldom moves, making it an easy target for its main predators, including the Sand Star, Luidia foliolata, and fish. In some areas, M. intermedia lives in densities of up to 15 per square meter, and individuals tend to aggregate in groups of two to six. Talk about a sea cucumber buffet!

M. intermedia in a sediment sample which has been partially rinsed through a sieve.

In each issue we will highlight one of the Sound’s many fascinating invertebrates. We’ll share details on identification, habitat, life history, and the role this critter plays in the sediment community. Can't get enough benthos? See photos from our Eyes Under Puget Sound collection on Flickr. Look for the Critter of the Month on our blog.

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